The present invention relates to an apparatus for applying a viscous or paste-like material through apertures in a stencil onto a substrate beneath the stencil and is particularly suitable for applying solder paste onto a board for producing a printed circuit board or for applying an adhesive or other bonding agent to a substrate via a stencil.
Control of the flow characteristics and composition of materials and the pressure thereof in the stencil printing onto substrates is very important if good results are to be achieved.
Difficulty has been hitherto experienced in balancing or controlling the higher pressure required for displacing solder paste from a reservoir to dispensing pressure relative to the lower pressure required for deposition at the actual stencil surface and it is with the resolution of such difficulties with which our aspect of the present invention is concerned.
With solder paste material and other materials which may contain volatile components it is important to confine the material within a relatively closed space to prevent evaporation and where the materials are thioxotropic or contain such additives, it is important to minimise any evaporation. It has now also been discovered desirable and advantageous to maintain a movement of the viscous or paste-like material, especially immediately prior to application so as to maintain relatively constant rheological characteristics of the soldering paste and it is with this object with which a first aspect of the present invention is concerned.
Recent attempts have been made to provide improved apparatus for application of solder paste such as in WO/96/20088 of Ford Motor Co. Ltd. (PCT/GB95/03002) wherein a compression head cap provides a contained environment to direct and to aid the flow of pressurised viscous material through openings (48) in the stencil. The compression head cap (8) includes two generally parallel wiper blades (78xe2x80x2,78) acting on the stencil with their end edges only that are in sliding communication with the stencil (46). Longitudinal diffusion plates (90) guide and equalise flow of the viscous material into the volume between the two generally parallel and upwardly extending wiper blades (78xe2x80x2,178) acting on the stencil with their end edges only. In this apparatus the viscous material is forced through the interior chamber (66) of the compression head (6) wherein it is diffused by a diffuser (90) with the material then flowing into the exit (68), and then on to the stencil surface. EP-A-0504501 Zimmer and U.S. Pat. No. 4,023,486 also disclose some of these features of Ford.
In WO 98/16387 Societe Novatec S. A. (PCT/EP97/05761) the apparatus of the Ford disclosure is described as not overcoming realised problems and possessing various disadvantages. Specification WO 98/16387 of Novatec discloses a hollow receptacle (7) in which a piston (10) is displaceable to urge soldering paste (9) through apertures in a grill (17) functioning as a homogenising element upstream of two inclined wiper members (15,16) and then between the upper members (15,16) and onto the stencil and through the apertures in the stencil. In this device there are no means within the hollow receptacle (7) above the grill (17) for creating a flow of paste material other than through the grill and, furthermore, a considerable amount of paste may remain in the space defined beneath the grill and partially encompassed by the wiper bladesxe2x80x94which may lead to a reduction in quality of paste or other material when the device is temporarily out of use. DE offenlegungschrift 2250092 discloses a similar arrangement to Novatec.
According to a first aspect of the present invention there is provided apparatus for use in a stencil printing machine which has a carriage for executing reciprocable movements or strokes, for applying a viscous or paste fluid material to the surface of a stencil and through apertures therein onto a substrate, said apparatus comprises a fluid material applicator device which is slideable along an upper surface of a stencil and has at least one aperture in the lowermost surface through which may flow the fluid material to be deposited on the substrate below, at least one fluid material reservoir or means for receiving such in fluid material flow-communication with a flow passage in said applicator device, and means operable for displacing the fluid material during each stroke of the stencil printing machine of an amount greater than necessary to supply fluid material to the at least one outlet aperture for deposition on the substrate via the stencil.
The greater amount of fluid material which is displaced in the applicator device often requires a significantly greater pressure than desirable for application of fluid material to the stencil, and it has been found desirable for a pressure reduction means, such as a partition with one or more apertures or other flow restriction means, to be provided prior to said at least one outlet and preferably between a main flow passage of the applicator device or head and a passage leading to such outlet. A significant reduction of pressure is often required, such as in the order of a ratio of 6 to 1, for example, in a reduction of say 30 p.s.i. to 5 p.s.i.
In a second aspect according to the present invention there is provided apparatus for use in a stencil printing machine and for applying a viscous or paste-like material or other fluid material through apertures in a stencil and onto a substrate beneath the stencil, comprising a fluid material applicator device which is slideable along an upper surface of a stencil in fluid retaining relationship therewith and has a main-flow passage in which fluid material may flow in alternate directions or through which fluid material may flow in alternate directions or in reciprocable manner or may flow intermittently interruptably or continuously in the same direction and also communicates with at least one aperture in the lowermost surface through which may flow the fluid material to be deposited on the substrate, and at least one fluid material reservoir or means for receiving such in fluid material ducting flow-communication with said main-flow passage, and means for exerting pressure on said fluid material alternatively from either end of said main flow passage preferably after each stroke of the printing machine, or means for exerting pressure intermittently or continuously in the same direction, with means preferably provided or available for accommodating the flow of fluid material resulting from said pressure application which is additional to the flow of material required for a deposition on the substrate.
Preferably two reservoirs will be provided one at either end of said main flow passage and preferably comprise two cartridges as piston cylinder arrangements and with the pistons alternately displaceable during each stroke of the stencil printing machine so as to drive fluid material in opposite directions, even if only for two or three millimeters, for example, to keep the level of the fluid moving during printing.
Preferably the lowermost surface of the applicator device is planar or substantially planar and the device is such as to enable the two pressure members of the printing machine, in use when the applicator device is located thereunder, to be urgeable against receiving means on the upwardly facing side of the device, so that the planar lower surface is urgeable against the upper surface of the stencil on which it is sliding and the pressure members alternate in application to be on the downstream side of the device with respect to the direction of displacement of the device and above the sealing edge of the at least one lowermost aperture.
In one embodiment, an elongate slot as said at least one aperture is provided for the passage of paste or other fluid although such may be multi-apertured such as in the form of a grill or mesh or the like, and preferably the sheet of material defining such will be integral and preferably the sheet of material will provide an uninterrupted or continuous non-apertured region surrounding the apertured region.
Preferably the applicator device will have two reservoir means connectable and/or connected to be in communication with the opposite ends of the ducting means or main flow passage and displacing means such as a piston may be provided for each reservoir means for enabling the fluid means in the free flow passage to be preferably alternately displaceable from a first reservoir means into the main flow passage and for some fluid already in the main flow passage to be displaced therefrom into a second reservoir and vice versa.
The present invention may be provided as a removable/replaceable apparatus for a screen printing machine and the features of design mentioned herein enable optimum utilisation of the paste with minimum waste. Preferably two reservoirs are provided, with each reservoir being detachably removable and replaceable with a fully charged like reservoir for speed and simplicity of screen printing.
The apparatus of the present invention may be fitted/retrofitted to existing screen printing machines whereby the previously used squeegee members are removed from the displaceable arms of the machine and the present apparatus located in position under and alternately held by means of the displacing arms as such are alternately reciprocated upwardly prior to horizontal displacement by the carriage of the printing machine. The displacement arms of the screen printing machine are normally alternately displaceable in upward and downward movement of the pressure members attached to the machine and shoulder or other arm-receiving means are provided on the present apparatus to receive the pressure which facilitates fluid sealing of the applicator device in sliding relationship on the stencil and also to enable the present device to be horizontally driven in alternative directions.
The reservoir is preferably of light material such as plastics material so as to possibly be disposable or rechargeable.
The apparatus of the present invention minimises the risk of viscous materials becoming stiff or otherwise unsuitable and also reduces amount of material which might be wasted and enables a rapid replacement of the reservoirs.
Also according to the present invention there is provided a pressure reducing and dispensing foil or foil members having in one embodiment three walls or surfaces triangularly disposed or disposable and comprising a lowermost, generally planar apertured wall having at least one aperture therein for the throughflow and deposition of paste onto a substrate, and two lateral walls extending from opposite elongate edge regions of said lowermost wall and extending upwardly and convergingly with at least one said lateral wall having one or more pressure reducing apertures provided therethrough, and preferably a plurality of such apertures spaced along each wall, and alignable with apertures in a lower support or retaining member. Preferably the foil will have end walls to close the ends of the defined passage.
Whilst the foil described above and illustrated in FIGS. 6A and 7 is one piece, for ease of changing the lowermost apertured foil part or member, such foil may be provided in four parts, an upper apertured pressure reducing part, two lateral retaining parts with retaining edges for holding the main lower defined foil member defining the lowermost paste exit aperture or apertures as will now be defined.
In a preferred embodiment, however, there is provided a pressure reducing foil comprising an apertured strip wherein the apertures when located in an applicator device act to reduce the pressure in the fluid before it flows to the outlet defined by a separate lowermost flexible foil member which preferably has a continuous peripheral portion defining the outlet aperture or apertures and in effect forms a fluid medium printing gasket. Thus in a simple form of this preferred outlet foil or printing gasket, there is provided an elongate flexible foil or metal sheet member having an elongate slot extending along its length and defined by the periphery or preferably detent means are provided for releasably engaging with shoulders as thin retaining foils clamped on the device. Preferably such retaining foils will have different fold lines so as to permit interchanging so as to provide for different positions of retention of the outlet foil. Alternatively, the lower apertured foil member may be held laterally in channels formed in the base of the applicator device instead of by lateral foil members.
Preferably resilient sealing means, such as a resilient ring, such as an O-ring will be provided in a groove to urge the lower foil member downwardly to seal against a stencil in use.
The present invention is intended to encompass such foils in flat unfolded form also.
Also according to the present invention a screen printing apparatus whenever adapted or modified to incorporate the apparatus with pneumatic control of the present invention.
It is also envisaged that an especially dedicated machine might be provided rather than modification of existing or retrofitting of existing machine.
Also according to the present invention a method of applying a viscous or paste or like fluid material through a stencil to a substrate comprises displacing more fluid material than is necessary for the application of the fluid material to the substrate for each operative application movement.
In a third aspect according to the present invention there is provided apparatus for use in a stencil printing machine and for applying a viscous or paste-like material or other fluid material through apertures in a stencil and onto a substrate beneath the stencil, comprising a fluid material applicator device which is slideable along and supportable by an upper surface of a stencil in fluid sealing relationship therewith and has a main flow passage which communicates with at least one dispensing aperture in the lowermost surface through which may flow the fluid material to be deposited on the substrate, and at least one fluid material reservoir or means for receiving such fluid material in ducting flow communication with said main flow passage, and means for exerting pressure on said fluid material in said reservoir, and pressure reduction means for reducing the pressure of the fluid material located between the dispensing aperture and said main flow passage.
The lower-pressure ducting means is defined in part by a pressure reduction and carrier member which does not reduce in cross section towards the sealing means or gasket but rather increases in transverse cross section.
Said at least one dispensing aperture and the pressure reduction means may be provided as separate parts each of which is detachably mounted on a support and replaceable with an identical component, if one is worn or damaged, or with a differently apertured component if a different fluid is to be applied. Also the pressure reduction means or part may be replaceable as desired and may be modified as desired by selecting appropriate aperturing on the component and the manner of deposition of the fluid material from the device determined by selecting the requisite size and/or shape and/or pattern or arrangement of the lowermost aperturing. This replaceable component may be formed by flexible sheet material or foil and is indicated on the accompanying drawings as an injector foil.
Preferably, the main housing or body of the applicator device according to the invention which defines the main flow passage and the reduced pressure flow passage interconnected by fluid material pressure reduction means is formed as one piece or integrally formed and preferably formed by extrusion, for example, in aluminium. The construction achieves considerable economy in manufacture and assembly and ease of use. In this one piece or integral construction, the pressure reduction means is preferably formed as a slot or elongate passage extending along preferably almost the whole of the length but terminating short of the ends.
This elongate pressure-reduction passage is advantageously formed by having the main body extruded with the main flow passage extending therealong, and preferably such that the lower sidewalls defining such converge (possibly somewhat in the manner of the section through the bottom of an inverted tear drop or water drop), and then machining in the lower face of the housing a reduced-pressure flow passage or recess, terminating short of the ends, and machining or cutting into the converging sidewalls defining the main passage to a sufficient extent and in a manner as desired to form an elongate slot or passage of a width to achieve the required pressure reduction. The elongate slot may be of constant cross-section or width or by varying the depth of machining may be of varying width, for example, may be narrowest near the inlet from the fluid reservoir or reservoirs and enlarge away therefrom to the middle and then converge again towards the other end and inlet of the other fluid reservoir when two such are provided. The varying of the width of the elongate slot is such as to distribute the fluid as evenly as possible along the slot and into the reduced-pressure flow passage
The outlet aperture defining member or printing gasket is preferably in the form of a flexible metal foil with a continuous peripheral portion whose inner edge acts as a fluid wiper and is preferably backed by a resilient member such as an O-ring. Preferably a spacing member surrounds or substantially surrounds the gasket and is preferably of a material, such as PTFE, which facilitates sliding of the applicator device on a stencil and also is one determining factor as to how much the gasket foil is flexed against the O-ring in use. The provision of the spacing means or member around the gasket enables a gasket with different length of aperture or apertures to be used and thus the device of the present invention can be used on substrates of different widths.
In a further aspect of the present invention there is provided a fluid applicator device comprising a cross print-head or body means defining fluid ducting means and in communication with connection means, provided for connection of a fluid reservoir means, and said ducting means leading to an outlet passage in the lower surface of the device slidable on a substrate, characterised by the feature that the outlet passage comprises an aperture or apertures through which fluid flows to be deposited onto and through a stencil therebeneath, is defined by a gasket or sealing means in the form of a thin sheet of material (such as a metal foil) having a continuous peripheral portion surrounding the paste outlet aperture or apertures with an outer portion of said sealing means being secured to the body means or part thereof (preferably located in two parallel containing grooves or channels extending parallel to a central elongate opening) and being supported on its upper surface against yieldable means or resilient means (such as a rubber O-ring located in a groove) and which, peripheral portions incline downwardly preferably only slightly downwardly out of the horizontal plane or plane of the base of the body; and when not subject to counter pressure, the aperture defining edge(s) extends beneath the lowermost plane of the body means in which the lowermost surface(s) of the body or any spacing means lie by an amount such that when the applicator device is supported on a surface (such as on top of a stencil), the inner part of the peripheral portion and sealing edge of the gasket or sealing means are flexed/deflected upwardly against the counter pressure of the resilient means (preferably a rubber O-ring) so as to provide a sliding seal for preventing or minimising the escape of fluid.
Preferably spacing means or members (or a xe2x80x9cshoexe2x80x9d) are provided located outwardly of the aperture edges of the sealing means or gasket either at opposite ends and/or along opposite sides although preferably being provided located extending beneath and along and around all sides of the peripheral portion of the sealing means and leaving a central space into which the aperturexe2x80x94defining edge(s) of the deflected portions of the sealing means or gasket extend. Thus the aperture-defining edge(s) of the gasket or sealing means extend beneath the plane in which the lowermost surfaces of the spacing means lie by an amount such that, in use, when supported on a stencil, the inner part of the peripheral portion of the sealing means is flexed upwardly against the resilient means so as to provide a sliding seal. The spacing means thus partially act to determine the pressure exerted by the gasket edges or sealing means.
Preferably the opposite ends of the gasket or sealing means will be pre-formed or pre-stressed so as to have a part-conical or hemispherical or other curved surface or other shaped surface to improve the sealing effect when flexed in use. Thus the resilient means e.g. an O-ring, acts on the broad face of an inner part of the outer continuous peripheral portion and thus does not act on the outermost end edge where the gasket or sealing means is secured.
Thus also according to the present invention there is provided a sealing means or gasket for an applicator device for solder paste or other fluid for use in stencil printing formed of an elongate, thin sheet of flexible material and comprising a continuous outer peripheral portion defining an inner apertured region for the passage of fluid, with opposite end regions of the continuous peripheral portion being pre-formed or shaped and pre-stressed to provide part conical end regions or an otherwise-curved or -shaped opposite end regions. This can be achieved by forming a sheet blank as a development of the desired resultant shape with straight parallel but inclined faces and of curved opposite ends and which necessitates in the flat, a straight side with opposite end regions running into a side which is divided and of inclined parts and then joining the parts to form the requisite part conical or other opposite end regionsxe2x80x94when the elongate sides are pressed parallel with the outer edges lying in the same plane so as to then become inclined.
Also according to the present invention there is provided a means for adapting a solder paste stencil printing machine having two reciprocable squeegee blades detachably mountable in receivers in two vertically and horizontally reciprocable carriers, comprising two elongate cross members securable by securing means in the receivers in the carriers and so as to extend parallel to each other, and a fluid applicator device having at least one projection slidably engageable by one or both of said cross members so as to enable said device to be reciprocably displaced upon operation of the machine. Preferably two projections extend from the device and are clamped between the cross members which are vertically slidable relative thereto. Spring means may be provided to maintain pressure of each cross member on the applicator device irrespective of its vertical position.
Also according to the present invention, a method of applying a viscous or paste or like fluid material through a stencil onto a substrate comprises reducing the pressure of fluid material prior to it being applied to the stencil and substrate.